Two titles in the Jake Samson/Rosie Vicente series by Shelley Singer are free right now at Kindle US and UK. One of them is the first one, which is nice if you like to read in order.
Samson's Deal, #1
Kindle US:
https://www.amazon.com/gp/product/B00DDU756Q/
Kindle UK:
https://www.amazon.co.uk/gp/product/B00DDU756Q/
Suicide King, #5
Kindle US:
https://www.amazon.com/dp/B00KKRHPJ2
Kindle UK:
https://www.amazon.co.uk/gp/product/B00KKRHPJ2/
None of the titles in this six-book series are very expensive, and they go on Countdown from time to time too, so if you're in luck if you decide you like these two. Also, there was an omnibus of all six that was briefly free a while back, so check carefully before you actually buy any...